[SiempreViernes]

Maybe it's because filtering by rating would expose how shallow the catalogue is?

At least in my locale a good way to be reminded of this is searching for any movie you'd like to watch but isn't on the front page of netflix, typically they won't have it.

[RobertoG]

I think you are right. My impression is that all the UI is dedicated to fool the user perception of the catalog, to make it look better than it is.